Electrical wave filter



Patented ll 27, 192i,

llREltT "W, DE MONTE, Uitl JERSEY CITY, AND TIM'THY' lil. fdl-llll., 0F RUTHERFORU,

NEW JERSEY, ASSIGNlltlil T WESTERN ELECTH'II COMlllltllll'Y, INCQRPORATED, IDIll- NEW YItFtK, N. Y., A. tlflltlilRAllUlll 013 `NEW YllM.

ELEGTRIGMA WAVE Application led October 7, 1924. Serial Nori. Hlfte.

yllhis invention relates to electrical wave filters, and more particularly to means hy which the edects of undesirable reaction loetween the sections of the filter may he avoided or reduced.

'lhe type of electrical wave filter to which this invention applies is that comprisingn a series of sections, each of which includes imn pedance elements so arranged as to attenulll ate certain frequencies and to transmit others without substantial attenuation. As each section is designed to transmit frequencies within a selected range of frequencies and more or less attenuate frequencies outside of lll this range, the accumulated effect of the suc cessive sections of the filter is to prevent certain frequencies from reaching the output 'terminals of the filter, while permitting the passage of others thereto. ln order to acl@ complish this result most edectively, it is desirable that the transmission of electrical energy he entirely between immediately adiacent points in the filter, that isz that energy, at least of the frequency desired tohe im suppressed, should not pass from one section of the hlter to other sections without umlern going the attenuating edect of the impedu ance elements of every section hetween the two points.. l-lowcver, since in general each il@ section of an electrical filtcr'contains an inductance element, there is" an opportunity for injurious reaction between sections of the filter through the external electromagnetic delds of these inductance elements.

This reaction or mutual inductance may loe suppressed to some extent hy the use of closed core coils for the inductance elements, but in general the interaction will still emst to some extent, and therefore to interfere t@ with the eflicient operation of the filter. 'loroidal inductance coils having practically no external magnetic fields have also been used, but coils of this type are expensive to wind, and have a high effective resistance in relation to their inductance, as compared with straight core coils of proper design. It is therefore often desirab e to use straight core nductances which can be cheaply constructed and which result in a small amount of power dissipation. Such inductances, however, have an inherently large stray magnetic field. 1n order to somewhat reduce the effeet of this stray magnetic held, the induen y tance coils of successwe sections have heen lid arranged in groups of three, with the axes of the coils of the group in the same plane and erpendicular to each other. This, however, as not heen found 'fully edective, and parn ticularly fails when. one of two correspondlingcoils of successive groups, whose aies are type, particularly those employing straight core inductances, by reducing the effects of mutualiinductanee between elements of different sections. Ilhis is accomplished by an arrangement of the inductances by which mutual inductance between selected nomad jacent sections of theA filter yis substantially suppressed',

In the accompanying drawings, Fig. l shows a simple low pass filter; Fig. 2 shows a low pass filter of the composite type; Fics. 3 and Ltare attenuation frequency curves or the lter shown in llig. l; and Figs. 5 and n show two methods of mounting the coils of 'the hlter shown in Figs. l and d, in accord ance with this invention.

The type of low pass electrical wave filter illustrated in Fig.. lv comprises a number of similar inductances L1, and L2, etc. in series and e 'ual ca -acities C1, C2, etc. in shunt thereof.. .lts is well known, each section of this type of lter comprising an inductance and capacity will freely transmit currents of a frequency below a certain value known as the cut-olf frequency and will more or less .attenuate frequencies above this value. Each section of the filter will also shift the phase of the transmitted frequencies. The currents above the cut-olf frequency will therefore decrease in strength from .sectionto section, the

total attenuation for the filter` depending upon the number vof sections, while the currents' below the cut-off frequenc will be transmitted in substantally un 'minished strength, although with a shifting of phase Afrom sectionto section. ,As noted above, it has been realized that the discriminating effectof the filter would be reduced bymutual inductance between adjacent coils, for in stance, L, and L since this would result in restoring to the frequency to be attenuated more or less of Ithe energy which the rst aection of the filter is intended to check, andin view of this the expedient has been adopted with some success of placing adjacent coils with their axes at right angles to each other.

According to the present invention, however, less attention in the arrangement of the straight core coils which constitute the inductanc'es L1, L2, etc., is paid to possible mutual inductance between adjacent inductance coils than to mutual inductance between coils moreor less widely separatr coils L1, L2 and La; while the last coil, L1',

is arrangedwith its axis perpendicular both` t'o the axes of L1, L2 and L3 and to the common axis of L4, L5 and Le. If a larger number vof coils were used, the same arrangement would be continued, that is, thel eighth and ninth coils would have their axes parallel to that of coil 7 while the axes of the tenth, 'eleventh and twelfth coils `would be parallel to the axes of the coils L1,

L and L3, which. would be comparatively widely separated in the filter from the tenth, eleventh and twelfth coils. In the arrangement of Fig. 5, coils L1, L, and L, may

-be regarded as a group of coils'widel separated in the filter, the coils L2 and 5 may be regarded as a secondl group of widel separated coils and La and La as a thir group. In each case it will be seen that the members of the group have their axes at substantially right angles to each other so that there will be no appreciable mutual inductance between the respective coilsof theA group. v

The beneficial effect of this arrangement on the functioning of the filter will perhaps :be more readilyunderstood by a consideration of Figs. 3 and 4.

In Fig. 3, curve A indicates the normal attenuation of the filter on the assumption" that there is no mutual inductance'between the coils of the different sections; curve C represents the results upon the attenuation of mutual inductance between coils L1 and L1 when the normal current at a given frequency in the two coils isin opposition; while curve D represents the attenuation when the current in the two coils is in phase.

It will be noted that although under the conditions of curve C the attenuation is greater than normal at some frequencies, yet both curve C and curve quickly alpproach an asym totic value indicated y line--B and that 4t is represents, for most of the frequencies it is desired to suppress, a substantially lower attenuation than the normal attenuation of the lter. Under these circumstances the efficiency of the filter will be much reduced for frequenciesl a short dis-V tance beyond the cut-off frequency.

In Fig. 4 there is represented a condition when there is mutual inductance between two widely separated coils other than end coils. Curve E represents the normal attenuation of the filter, G the attenuation when the coils under consideration are in (Iipposition for the particular frequency, and

the attenuation when these currents are in phase. The broken line F represents the asymptotic value to which the curves Gand H approach. While the peaks in attenuation due to mutual inductance under the conditions represented by Fig. 4 are less than in Fig. 3, still it will be seen that the loss in efficiency is substantial.

By the present invention the curves C and D in-Fig. 3 and curves G and H in Fig. 4 may be made to approach more or less closely the normal curves A and E, that is to say, the normal efficiency of the filter is substantially restored.

The mutual inductance between the coils may also reduce somewhat the strength of the currents which the filter is designed to ytransmit by bringing in opposition through phase differences the currents in different sections. The seriousness of this effect depends upon the phase differences of the currents in the coils involved, and this in turn, as notedabove, depends upon the separation i of the coils along the line of the filter. This undersirable effect is also substantially reducedl by the present invention.

Fig. 2 represents a modification of the filter of i 1, in which inductance L,l and capacity Gg1 nant to one-of the frequencies it is desired particularly to suppress, L2 and C2 are anti-resonant to another frequency it is desired particularly to suppress, andA the remaining sets of inductance and capacit elements in parallel therewith are similar y adjusted to resonate at a selected frequency to be sup ressed. In this arrangement, each section of) the filter will to some extent attenuate all frequencies above the cut-off freuency but will be particularly efficient at t e frequency at which its parallel arrangement o inductance and capacity is adjusted to resonate. The attentuation throughout the whole band beyond the cut-off frequency will ,therefore be made substantially greater and 'the' cut-0E sharper than in the case of the filter in Fig. 1.

The'present invention is particularly effective in suppressing the detrimental eect of mutual in uctances between coils of the filter of Fig. 2 where L1 and Cl for instance, resonate Vat a .frequency above the cut-off frequency. It has been found in this case that mutual inductance between L1 and L', results in a. very considerable annulment comprise a circuit anti-reso-` frequency is concerned. r"

of the attenuation etect of the filter at .the tr uency for which thel circuit LQ, C', is anti-resonant; the intervening sections of the lter may be in effect very effectively shunted so far as the attenuation of this particular may require a modification of the arrangement shown in Fig. 5. Fi 6 shows one ex. ample oi such a modi ed arran ment. This arrangement is adapte-d for a ter 1n which the coils L2 and L3"resonate near `the resonate at a frequency near the cut-off frecut-oi fre uency. It will be noted that the axes of ot er coils widely separated from coils L2 and'Ls in the filter, such as L,Ls and L7, are at an angle'of low mutual inductance to the axes of coils L2 and L3, and therefore undesirable inductive` effects 'from these two coils will be minimized, or greatly suppressed.` Mutual `induction between a givenpair ot coils comparatively remote om each other in the network, such as L1 and L4, for instance, may beof minor im ortance, in which case these'two coils may ave their axes arallel. The arrangement in Fig. 6 is mere y by way of example, and other modified arrangements of the inductance coils will, with the above explanation of the invention, be apparent to those skilled in' the art.

It 4'is obvious that although the invention has been particularly described as embodied in low pass filters, itiet it is equally applicable to filters of ot er types.

Y What is claimed is:

l. The method of improving thel operation et a multi-'section electrical network having elements capable of interaction by their external magnetic fields, which comprises reducing to agreater extent the interaction be# tween elements electrically more separated than the interaction between .adjacent elements.

2. The method of improving the cperation of multi-section electrical networks having inductance coils in a plurality of sections capable of interaction .by their external magV netic fields,` which comprises reducing the mutual inductance between the coils in sections electrically remote from each other te a greater extent than between Vadj acent coils. 3. The methodof improving the operation of a multi-section .electrical network havin an inductance coil in each section, whic co'mprises positioning said inductance coils inY space with more' consideration of sub stantial avoidance of inductive interaction 4between coils widely electrically separated in the network thanv of reventing inductive interaction between colis of electrically adjan't sections. y

4. Av wave filter comprising a plurality of sections, each containing an inductance element.one of said sections being'a'djusted to quency cf the wave lter, said section `having its inductance element arranged in a noninductive relation to the inductance elements of sections electrically remote therefrom in the network of the filter.

f 5. A wave filter comprisingla plurality of sectins, each containing a so enoidal inductance element one of said sectionsI being adjusted to resonate at a frequency near the cut-off frequency of the wave filter the'inductance element of said section belng particularly arranged to have no inductance mutual with elemente of sections electrically remote therefrom in the filter.

6. In a wave selective network of the type having a plurality of sections, solenoidal inv ductance coils formin a roup of more than. three in` number, wherey mutual inductances between certain coils are unavoidable, the coils of said group bein arranged with their centers in line, the pA ysical sequence corresponding generali to the electrical -sequence, and being divi edinto three succes: 'sive sub-groups, the coil axes in each subgroup being parallel'to each other and perpendicular to the coil axes in each other subgroup.

In witness'whereof, we hereunto' subscribe our names this 2nd day of October A. D., 1924.

, ROBERT W. DE MONTE'.

TIMOTHY E. SHEA. 

